1) Field of the Invention
The present invention relates to a feed-forward amplifying device suitable for radio equipment arranged in a base station in radio telecommunication systems such as digital automobile telephones, a method for controlling the same, and a base station with the feed-forward amplifying device.
2) Description of the Related Art
In the radio equipment of a base station in a radio communication system such as digital automobile telephone system, a multicarrier (a signal including plural frequency components selected every mobile unit at a used frequency band) is amplified in common for transmission to mobile units.
In a general amplifier used in the above-mentioned amplifying device, it is regarded that the output signal level increases linearly at a low input signal level, as shown in FIG. 12. As the input signal level, however, increases, the non-linear characteristic becomes noticeable (at the level exceeding the point a, for example, shown in FIG. 12).
Where the common amplification is performed in the above-mentioned radio equipment, a low-distortion amplifying device is particularly required because the intermodulation distortion component interferes with other channels.
FIG. 13 is a block diagram illustrating a transmitter equipped at a base station accommodating plural mobile units in a radio communication system such as a digital automobile telephone system. The transmitter 200 shown in FIG. 13 can amplify and transmit in common, for example, 12 kinds of frequency signals.
In the transmitter 200, numeral 201 represents a demultiplexer (DMUX) which separates a transmission signal every carrier; 202-1 to 202-12 represents a direct modulating unit that modulates each duplex carrier and subjects a high-frequency signal to a frequency conversion; and 203 represents a hybrid circuit (H) that combines signals from the direct modulating units 202-1 to 202-12 together.
Numeral 206 represents an amplifying device. The amplifying device 206 amplifies in common a transmission signal as a multicarrier signal sent to the mobile units 209 accommodated in the transmitter 200. The amplifier 206 includes two amplifiers 204-1 and 204-2 and switches 205a to 205d.
Two amplifiers 204-1 and 204-2 receive an input signal with a level in the area A shown in FIG. 12, with a fixed de power supplied thereto, and amplifies and produces it with a fixed amplifying factor according to the de power. A control unit (not shown) on/off controls the switches 205a to 205d according to the power of the transmission signal output from the hybrid circuit 203.
In concrete, when the power of the transmission signal from the hybrid circuit 203 is low, only the amplifier 204-1 is operated by turning on the switches 205a and 205b and turning off the switches 205c and 205d. When the power of the transmission signal from the hybrid circuit 203 is high, both the amplifiers 204-1 and 204-2 are operated by turning on all the switches 205a to 205d.
Numeral 207 represents a transmit/receive shared unit that outputs a transmission signal from the amplifying device 206 to the antenna 208 arranged at the rear stage. The signal from the mobile unit 209 received by the antenna 208 is transmitted to the receiving system (not shown) via the transmit/receive shared unit 207.
In the transmitter 200 shown in FIG. 13, the amplifying unit 206 amplifies a multicarrier signal carrying 12 kinds of frequency signals according to the signal power and then transmits the result. The amplifiers 204-1 and 204-2 amplify linearly the transmission signal according to the input signal level so that an intermodulation distortion component such as amplitude change and phase shift is suppressed.
The dc power supplied to the amplifiers 204-1 and 204-2 is sufficiently larger than the output signal power. The heat dissipation fin (not shown) equipped to the amplifiers 204-1 and 204-2 converts the residual component being a difference between the supplied power and the signal power of the amplifiers 204-1 and 204-2 into heat energy and radiates it.
With an input signal in the level of the area A shown in FIG. 12, since an output signal is low in level, the dc power supplied cannot sufficiently converted as a power energy. Hence there is a problem in the power use efficiency.
On the other hand, it may be considered to use the transmitter 200A shown in FIG. 14 including the amplifying device 206A formed of a sole amplifier 204A. The reason is that the supplied dc power can be sufficiently used as power energy by inputting the input signal to the amplifier 204A in the vicinity of the saturated level as shown with the region B in FIG. 12 even when the signal output of the hybrid circuit 203 is small.
However, in the transmitter 200A shown in FIG. 14, the problem is that the intermodulation distortion component such as amplitude change or phase shift becomes large since the amplifier 204A amplifies non-linearly a transmission signal as shown with the region B in FIG. 12.
Moreover, it is considered to suppress the intermodulation distortion component such as amplitude change or phase shift by subjecting the feed-forward amplifying device including a sole amplifier shown in FIG. 15 to a feed-forward control.
The feed-forward amplifying device shown in FIG. 15, which can be equipped to the amplifying device 206A of the transmitter 200A in a base station which accommodates plural mobile units in a radio communication system such as a digital automobile system, shown in FIG. 14, can amplify in common the transmission signal to mobile units 209 as a multicarrier signal.
Referring to FIG. 15, numeral 101 represents a distortion extracting loop circuit unit. The distortion extracting loop circuit unit 101 extracts a distortion component included in the main signal output from the main amplifier 104. The distortion extracting loop circuit unit 101 includes the branching unit 102, the variable phase shifter and variable attenuator 103, the attenuator 105, the delay line 106, and the synthesizing unit 107, in addition to the main amplifier 104.
The branching unit 102 branches the main signal at the previous stage of the main amplifier 104. The variable phase shifter and variable attenuator (PSV1/AV1) 103 varies the phase and amplitude of a main signal branched by the branching unit 102 according to the control signal from the CPU 115 (to be described later) and then produces the result to the main amplifier 104.
The attenuator 105 receives the main signal amplified by the main amplifier 104 and a pilot signal supplied from the pilot signal generating unit 100 and then attenuates them to the level before they are amplified. The delay line 106 delays another main signal branched by the branching unit 104 by a predetermined period of time. The synthesizing unit 107 synthesizes a signal from the attenuator 105 with the signal from the delay line 106 and then produces the result as a distortion extracting signal.
Numeral 108 represents a distortion removing loop circuit unit. The distortion removing loop circuit unit 108 produces only the main signal component of the signal in the main signal system at the rear stage of the main amplifier 104, using the distortion extracting signal obtained by canceling the main signal component from the distortion extracting loop circuit unit 101. The distortion removing loop circuit unit 108 includes the delay line 109, the variable phase shifter and variable attenuator 110, the auxiliary amplifier 111, and the synthesizer 112.
The delay line 109 delays a signal amplified by the main amplifier 104 by a predetermined time. The variable phase shifter and variable attenuator (PSV2/AV2) 110 varies the phase and amplitude of the distortion extracting signal from the synthesizing unit 107, based on the control signal from the CPU 115 (to be described later).
The auxiliary amplifier 111 amplifies the distortion extracting signal from the variable phase shifter and variable attenuator 110. The synthesizing unit 112 synthesizes the signal from the delay line 109 with the signal amplified by the auxiliary amplifier 111 and then produces the result as a distortion removing signal (main signal).
Numeral 113 represents a detector which receives the distortion extracting signal output from synthesizing unit 107 and detects the main signal component included in the distortion extracting signal. Numeral 114 represents a detector that detects the distortion signal component (a component other than the main signal component) included in the distortion removing signal input from the synthesizing unit 112.
The CPU 115 controls the variable phase shifter and variable attenuator 103 according to the detection level of a pilot signal of the detector 114 to convert the signal output from the synthesizing unit 107 into a distortion extracting signal of which the main signal distortion signal component is most canceled (or canceled maximumly), and controls the variable phase shifter and variable attenuator 110 to convert the distortion removing signal output from the synthesizing unit 112 into a main signal of which the distortion signal component is best canceled (or canceled maximumly).
In the feed-forward amplifying device having the configuration shown in FIG. 15, the distortion extracting loop circuit unit 101 receives a multicarrier signal and produces a distortion extracting signal of which the main signal component is canceled maximumly, under the control of the CPU 115. The distortion removing loop circuit unit 108 produces the main signal of which the distortion signal component is canceled maximumly, under the control of the CPU 115.
Where the multicarrier signal is stably input to the feed-forward amplifying device, the phase and amplitude in the variable phase shifter and variable attenuators 103 and 110 can be controlled in a balanced state.
However, the multicarrier signal input to the general feed-forward amplifying device may be instantaneously produced at a peak power several times the average power. In the case of, for example, 8 multicarriers, the maximum value of the peak power increases instantaneously by about 8 dB, for example, to the average power.
The problem is that the loop circuit units 101 and 108 are unbalanced due to the nonlinear characteristic such as the AM-PM characteristic of the amplifier when the multicarrier signal is input at a peak power to the feed-forward amplifying device, whereby the distortion component of the output signal from the feed-forward amplifying device increases.
FIG. 16 is a diagram showing an example of the AM-PM characteristic of a general amplifier used in the main amplifier 104 or the auxiliary amplifier 111 mentioned above. FIG. 17 is a vector diagram showing an example of the characteristic shown in FIG. 16. Referring to FIG. 16, Pa represents an average input power of a multicarrier signal to the main amplifier 104 and .theta.a represents a phase rotation of the main amplifier 104 at the power Pa. Hence, as shown, for example, in FIG. 17, the average input power (vector length) Pa and the phase rotation .theta.a can be represented as the vector Pa.
In FIG. 17, the vector is shown with an arrow and the information on a vector length is represented with an absolute value of a vector. However, it should be noted that the notation is not limited only to this example.
Pp represents a peak level of a multicarrier signal and .theta.p represents a phase rotation of the main amplifier at the power Pp. Hence, as shown in FIG. 17, the peak level (vector length) Pp and the phase rotation .theta.p can be represented as the vector Pp. In this case, it is assumed that the distortion extracting loop circuit unit 101 which cancels the main signal is in a balanced state to the average input power Pa.
If the power of an input signal input to the main amplifier 104 is less than Pa, as shown in FIG. 16, the phase of the amplified signal acting as an output signal is near to .theta.a, or does not nearly vary. When the power of the input signal is at the peak level Pp, the phase of the amplified signal acting as an output signal varies to .theta.p, or rotates by .DELTA..theta.=.theta.a-.theta.p.
In this case, it is difficult to deal with the phase rotation .DELTA..theta. of the amplified signal under the phase control of the variable phase shifter and variable attenuator 103. The cancel amount (the amount of the main signal component canceled) of the main signal due to the distortion extracting signal output from the distortion extracting loop circuit unit 101 decreases corresponding to the amount of .DELTA..theta.. As a result, the main signal residual component at a peak time is the vector Pp-a [=vector Pp-.alpha.(Pp/Pa).multidot.vector Pa] (where .alpha. is a coefficient representing the AM--AM characteristic of the main amplifier), for example, shown in FIG. 17.
The distortion extracting signal including the main signal component not canceled is input to the auxiliary amplifier 111 via the variable phase shifter and variable attenuator 110. Like the main amplifier 104, the auxiliary amplifier 111 varies its pass phase because of the non-linearity of the AM-PM characteristic.
It is difficult that the variable phase shifter and variable attenuator 110 deals with the phase variation of the amplified signal output from the auxiliary amplifier 111 so that the distortion removing loop circuit unit 108 is unbalanced. Moreover, since the distortion caused in the auxiliary amplifier 111 becomes an unignorable value, the distortion component included in the main signal acting as an output signal of the feed-forward amplifying device increases.
In other words, at a peak power time when an input multicarrier signal occurs instantaneously, it is difficult to make the distortion extracting loop circuit unit 101 and the distortion removing loop circuit unit 108 follow controlling the amplified signal. Thus the distortion component of the output from the feed-forward amplifying device increases because of deterioration of the distortion component canceling amount.
In order to avoid the distortion of an amplified signal at a peak power of a multicarrier signal, it may be considered that a point where the effect of the AM-PM characteristic does not extend (or where the power of an input signal is less than Pa) is selected as the operational point of the amplifier. However, this approach leads to a large back-off by which the amplification efficiency of the amplifier is decreased.